1. Field of the Invention
The present invention relates to an image processing apparatus, such as a copy machine or a laser printer, that forms an image, and a method for forming an image.
2. Related Background Art
One of the conventionally known methods (image processing methods) for the control of the image processing characteristic of an image forming apparatus, such a copy machine or a printer, is as follows.
First, an image processing apparatus is activated and warmed up, and then a specific pattern is formed on an image holder, such as a photosensitive drum. The formed pattern is read, and, based on the density value of the read-out pattern, the operation of a circuit, such as a .gamma. compensator (.gamma. conversion circuit), that determines image forming conditions is altered, so that the quality of a produced image is stabilized.
When the gradation characteristic is changed by the fluctuation of environmental conditions, the specific pattern is again formed on the image holder, read out, and refed to a circuit, such as the .gamma. compensator, that determines image forming conditions, so that a constant image quality can be maintained that is in consonance with the changed environmental conditions.
In the above conventional example, however, when an image processing apparatus has been in use for an extended period of time, the read-out density value of the pattern on the image holder sometimes does not correspond to the density of the image that is actually printed out.
Over time, the surface of an image holder becomes worn by the constant rubbing it receives by being in contact with a cleaning blade, for example, that is employed to remove transfer residual toner. Accordingly, the relationship between the adhering toner amount and a reflected light amount differs from that which exists in the initial state.
When, therefore, an image processing apparatus that has been in use for an extended period is returned to the state for image forming by employing density data that are acquired by using an initial density conversion parameter, an optimal image cannot be obtained.
Further, in the above conventional example, since no thought is given to the deterioration of the maximum image density for an image processing apparatus, when the output of the maximum image density is reduced due to a change in durability, etc., the gradation of an image is adversely affected within the high image density range no matter how the .gamma. characteristic is corrected.
In addition, since the gradation characteristic in the image processing apparatus in the above prior art is not linear (in particular, the characteristic for highlighting is not linear), the density that is obtained by interpolating density data by using an expression for an approximation differs from the actual density. When the gradation data is fed back to the image forming apparatus, an optimal image cannot be produced.
Moreover, in the above prior art, when an uneven density condition occurs that is due to a charging variance, because a charging unit is not clean, even though an image processing apparatus outputs an image with a uniform density across the entire surface of a recording medium, the density is varied for an image that is actually produced with the same density output. Under these conditions, if the gradation data is fed back to the image processing apparatus, an optimal image cannot be provided.